add clang-format to CI

- this will run clang-format on all changed C/C++ files
- list of changed files is taken from
https://forum.gitlab.com/t/ci-cd-pipeline-get-list-of-changed-files/26847/18
- loosely based on okular setup

.gitlab-ci.yml

+variables:   
+  DEBIAN_FRONTEND: "noninteractive" 
+
+include:   
+  - https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-before.yml   
+  - https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-applications-linux.yml 
+
+clang_format:   
+  stage: build   
+  image: debian:testing   
+  only:     
+    - merge_requests     
+    - master   
+  before_script:     
+    - apt-get update     
+    - apt-get install --yes --no-install-recommends git clang-format-11   
+  script:     
+    - wget -c --output-document=.clang-format https://invent.kde.org/frameworks/extra-cmake-modules/raw/master/kde-modules/clang-format.cmake
+    - git diff-tree --no-commit-id --name-only -r origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME -r $CI_COMMIT_SHA | grep -v -P '[.](c|cc|cpp|hpp|h)$'| xargs clang-format-11 -i
+    - git diff --exit-code